This competing continuation grant is a request to support further analysis of human age-related nuclear cataracts, the most common form of human cataract that causes visual decline and worldwide blindness. Surgery is the only current treatment, and, while very successful, is a significant burden on healthcare systems and does not address the high incidence of nuclear cataract in aging populations. The long-range goals are to understand the cell biological changes that produce excess light scattering in lens nuclei so that alternative strategies can be devised to prevent the onset or slow the progression lens opacification. The emphasis is on the role of fiber cell junctions in cataract formation. Important progress in the previous grant period includes the identification of 1-4 5m diameter particles that appear to be multiple thin lipid bilayers surrounding nearly spherical globules of cytoplasmic protein. These particles were detected by confocal, multiphoton and bright field light microscopy and by electron microscopy. The particles were found to be more numerous in nuclear cataracts than age-matched transparent donor lenses. A method of quantitative electron absorption was developed to measure the local protein concentration and refractive index of particle interiors compared to adjacent cytoplasm. The ratio gives a measure of the scattering potential. Using Mie scattering theory, these globular particles are potentially very efficient scattering centers producing forward scatter that could affect image formation at the macula. Ultrastructural analysis also demonstrated numerous changes at fiber cell junctions including loss of membrane components, low-density regions (voids), enlargement of extracellular spaces and protein deposits within extracellular spaces. These changes are probably related to protein modification and redistribution during aging and cataract formation. Texturing of the cytoplasm was observed and is probably based on the formation of high molecular weight aggregates. Fourier analysis was used to relate the amount of texturing to the observed opacification of the nucleus. These features appear to be enhanced, including the number of multlamellar particles, in advanced nuclear cataracts from patients in India. Future studies include the characterization of the amount predicted scattering from various types of cellular damage over the entire range from transparent donor lenses to fully opaque nuclear cataracts, thus leading to the mechanism of cataract formation. PUBLIC HEALTH RELEVANCE Cataracts are very common throughout the world and millions of surgical extractions are performed each year to correct visual deficits and blindness caused by cataracts. Surgery at present is the only effective treatment. Our goal is to understand how cells in the lens change during aging and cataract formation so that alternative strategies for prevention and cure can be devised and evaluated.